ISO's OSI Layered Model

The Open Systems Interconnection Reference Model (OSI Reference Model or OSI Model) is an abstract description for layered communications and computer network protocol design. It was developed as part of the Open Systems Interconnection (OSI) initiative. In its most basic form, it divides network architecture into seven layers which, from bottom to top, are the Physical, Data-Link, Network, Transport, Session, Presentation, Application Layers (Please Do Not Throw Sausage Pizza Away). It is therefore often referred to as the OSI Seven Layer Model.

Control is passed from one layer to the next, starting at the application layer in one station, proceeding to the bottom layer, over the channel to the next station and back up the hierarchy. Data exists at each layer in units called Protocal Data units (PDU). The picture gives the PDU at each layer.
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